Exponential aperture distribution horn antenna

ABSTRACT

A horn antenna suitable for use at microwave frequencies having an exponential amplitude distribution across substantially the entire aperture is composed of three sections. The first section is a section of quarter-wave long dielectric-filled waveguide which matches the waveguide wave number (or propagation constant) to the propagation constant at the aperture required for a damping factor which will yield a desired boresight scale factor at the design wavelength. The second section is also a dielectric-filled waveguide which supports a complex wave including a transverse standing wave in a central dielectric region and damped fields in the two air regions on opposite sides of the dielectric region, and whose propagation constant is the same as that at the aperture. The third section terminates at the aperture, and precisely at the aperture, the microwave fields have the characteristics of waves supported by a surface wave structure, the third section transposing the energy so as to consist of a transverse standing wave of very thin finite dimension in the dielectric material near a reference plane, and an exponentially damped wave (or hyperbolic cosine field) progressing from the reference plane, thereby to achieve an exponential aperture distribution across substantially the entire aperture. In other embodiments, the sections are combined.

United States Patent [191 Soong EXPONENTIAL APERTURE DISTRIBUTION HORN ANTENNA An-Hwa Soong, Westport, Conn.

United Aircraft Corporation, East Hartford, Conn.

Aug. 2, 1973 Inventor:

Assignee:

Filed:

Appl. No.:

US. Cl 343/776, 343/783, 343/785 Int. Cl. H01q 13/00 Field of Search 343/776, 783, 785, 786

[56] References Cited UNITED STATES PATENTS 3,392,396 7/1968' Ehrenspeck 343/785 Primary Examiner-Eli Lieberman Attorney, Agent, or Firm-M. P. Williams 5 7 ABSTRACT [111 3,831,177 [451 Aug. 20, 1974 three sections. The first section is a section of quarterwave long dielectric-filled waveguide which matches the waveguide wave number (or propagation constant) to the propagation constant at the aperture required for a damping factor which will yield a desired boresight scale factor at'the design wavelength. The second section is also a dielectric-filled waveguide which supports a complex wave including a transverse standing wave in a central dielectric region and damped fields in the two air regions on opposite sides of the dielectric region, and whose propagation constant is the same as that at the aperture. The third section terminates at the aperture, .and precisely at the aperture, the microwave fields have the characteristics of waves supported by a surface wave structure, the third section transposing the energy so as to consist of a transverse standing wave of very thin finite dimension in the dielectric material near a reference plane, and an exponentially damped wave (or hyperbolic cosine field) progressing from the reference plane, thereby to achieve an exponential aperture distribution across substantially the entire aperture. In other embodiments, the sections are combined.

6 Claims, 11 Drawing Figures BACKGROUND OF THE INVENTION l. Field of Invention This invention relates to antenna structures, and more particularly to a horn antenna having an exponential amplitude distribution across substantially the entire aperture.

2. Description of the Prior Art In a commonly owned copending application, Ser. No. 385,205, filed on even date herewith by B. R. Cheo et al, a phase interferometer antenna system which is capable of unambiguous detection of the angle of incidence of waves for angles of on the order of :t40 is composed of two antennas disposed in back-to-back relationship about a common reference plane, each antenna having an exponential aperture distribution over substantially its entire aperture. Certain classes of antennas ideally provided such an aperture distribution, including surface wave antennas and leaky wave antennas; however, it is difficult to approach ideal antenna characteristics with these classes of antennas. Another type of antenna which may be combined in pairs in accordance with the teaching of the aforementioned copending application comprises an array of a large number of closely spaced individual radiating elements; such an antenna is, however, extremely expensive and cumbersome, and provide less than ideal characteristics unless great care is taken in the design and fabrication thereof.

The utilization of antennas having exponential aperture distributions as set forth in the aforementioned copending application is exemplary merely. There are'additional applications in which the use of exponential aperture distribution antennas is highly advantageous, such as where the wave characteristics of a surface wave antenna are desired, but the structural features thereof are unsuitable to the intended environment, or where high power or efficiency are design criteria.

SUMMARY OF INVENTION The object of the present invention is to provide an improved antenna having an exponential amplitude distribution across substantially the entire aperture.

According to the present invention, a horn antenna having an exponential amplitude distribution across substantially its entire aperture comprises means including a dielectric member for converting, for instance, from a waveguide TE mode to a mode having standing waves in a centraldielectric portion with damped exponential (or hyperbolic cosine) waves on either side thereof, and for concentrating most of the energy in the dielectric portion and an exponentially dampedwave to one side of the dielectric portion, so that the horn has an exponential amplitude distribution across substantially the entire horn aperture. In further accord with the invention, the means including a dielectric member may provide for matching the'feed waveguide wave number (or propagation constant) to a wave number of the aperture of the antenna (with'the aperture having desired characteristics similar to those of a surface wave antenna).

In accordance still further with the present invention, an interferometer antenna system comprises a pair of antenna structures in accordance herewith, thereby to provide an antenna system having a combined aperture distribution which is exponential from boresight to a normal off boresight in one direction, and is exponential from boresight to a normal off boresight in an opposite direction.

The present invention provides a practical solution to the need for exponential aperture: distribution antenna structures. Antennas in accordance with the present invention may be fabricated utilizing techniques and materials which are well known and readily available.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partially broken away, sectioned side elevation view of an antenna structure in accordance with the present invention;

FIG. 2 is a plot relating dielectric thickness at the aperture and relative dielectric constants;

FIG 3 is a plot of an ideal exponential aperture distribution;

FIG. 4 is a plot of a distribution;

FIG. 5a is a diagrammatic section taken on the line 5-5 of FIG. 1;

FIG. 5b is a diagram of an equivalent transverse transmission line;

FIG. 6a is a diagrammatic section taken on the line 6-6 of FIG. 1;

FIG. 6b is a diagram of an equivalent transverse transmission line;

FIG. 7 is a schematic illustration of an interferometer antenna system employing two antennas of the type illustrated in FIG. 1;

FIG. 8 is a plot of sensitivity or scale factor of an antenna system of the type shown in FIG. 7; and

FIG. 9 is a sectioned side elevation of an alternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As an aid in understanding the major precepts of the present invention, a design technique is described in which the separate functions of the antenna structure are treated independently. Thereafter, variations therein are described.

Consider the antenna structure .20 disclosed in FIG. 1. This structure is designed to operate at about 16.5 CH2 and to provide an antenna structure which, when combined in a back-toback fashion with a like antenna structure sharing a common ground plane (as de scribed with respect to FIG. 7 hereinafter), will funcpractical exponential aperture tion as a phase interferometer antenna having a designed boresight scale factor (as described more fully with respect to FIG. 8 hereinafter) of ten electrical degrees per degree of angle of incidence. The microwave energy feed, at the left-hand end of the structure, is a rectangular waveguide operating in TE mode with the E-field also in the direction perpendicular to the sheet (in FIG. 1), and it is uniform in this direction. There fore, the present example can be treated as in a twodimensional problem. The antenna, in this example, is designed to workwith a WR-Sl waveguide, which is just over halfan inch high (in the Y dimension as seen in FIG. 1), and just over a quarter inch deep (into the sheet, as seen in FIG. 1). The aperture 21 of this antenna structure (the right-hand end of FIG. 1) comprises a ground plane 22 with an adjacent dielectric sheet 24 followed by a substantially infinite space 26. This profile at the aperture 21 has the same crosssection as that of a surface wave structure, and has the characteristics of a surface wave structure so that known techniques for analyzing surface wave structures and properties may therefore be utilized to define the end conditions at the aperture 21 of the antenna. For these design considerations, reference may be had to 1) Zucker, Francis J Surface And Leaky Wave Antennas, Chapter 16 of Antenna Engineering l-Iandbook (Henry Jasic, Editor), McGraw-Hill, 1961 (Library of Congress Card No. 59-14455), and by 2) IRE Transactions On Antennas And Propagation, Volume Ap-7, Dec. 1959, Special Supplement, Pgs. 8132-8296.

A surface wave structure provides an exponential field distribution above the wave-supporting surface. In the theoretical design of a horn antenna in accordance with the present invention, the design assumes that a perfectly exponential aperture distribution across the entire aperture is to be achieved, and utilizes parameters required for such a result, as defined by surface wave analytical technology. The purely exponential amplitude distribution as defined as one in which the amplitude of the field is of the form given by where A, maximum aperture amplitude,

e Naperian operator, y =distance along aperture from the reference plane,

and oz attenuation constant of aperture distribution, a property of the dielectric material and thickness. The constant a is a design parameter; its actual value depends on a particular application. For example, in an interferometer application, equation (14) of the aforementioned Cheo et al application defines the sensitivity or scale factor at boresight (BSF) as BSF 2k /z where k, 211%,, and A, wavelength in space;

at 16.5 GHz, A, z 1.82 cm, so

k 3.456/cm In the design example (an interferometer) being considered, it is desirable to have a high sensitivity or scale factor at boresight, in order to overcome noise and other signal definition problems. Thus, a BSF of on the order of ten electrical degrees per degree of angle of incidence (as described with respect to FIG. 8, hereinafter) may be appropriate in antennas having exponential distributions used in an interferometer antenna system (as described more fully with respect to FIG. 7 hereinafter), of the type described and claimed in the aforementioned Cheo et al. application. From equations (2) and (3), as BSF of ten will require a 2k /l0 0.693/cm.

From this, the longitudinal wave number k,, or propagation constant along guide direction, of the dielectric material 24 can be determined from WW) 3.525/cm.

a -k, cot (k,d)

where k, the transverse wave number in the dielectric and is related to the longitudinal wave number by The relationship between thickness and relative dielectric constant required to achieve the design parameters of equations (2)-(6) is plotted in FIG. 2.

The choice between a thin dielectric with a high relative dielectric constant or a thick dielectric with a low relative dielectric constant must be made. If the dielectric is too thin, it will not have structural integrity; on the other hand, the thicker the dielectric material, the more the aperture distribution will digress from a pure exponential distribution. Also, if a very high relative dielectric constant is chosen, then minor changes in the thickness will result in a large alteration in the characteristics of the antenna, which can result in significant lack of reproducibility in a production run of antennas.

For purposes of illustration herein, a material having a relative permitivity of 4.0 is assumed. .This results in a thickness of the dielectric 24 at the aperture of 0.28

cm. This material may, for example, be that sold by Custom Materials, Inc., Chelmsford, Mass, under the designation 707L-4, or other suitable material.

The overall height of the aperture should be large when the attenuation constant, a, is very small (sothat the amplitude drops off only slowly with distance along the aperture in the Y direction), and on the other hand, can be truncated to some extent if a is large (so that substantially all of the energy is contained within the distribution fairly close to the reference plane 22). Also, the smaller the wavelength, the smaller the aperture may be, subject to consideration of the attenuation constant. Several waveguide wavelengths will usually suffice. In this particular example, an aperture on the order of 3 to 4 inches (in the Y direction, or upward in FIG. 1) from the reference plane 22 is adequate.

Thus the conditions and design parameters of the aperture have been fully defined.

Next, it is desirable to consider the end conditions at the feed end of the antenna structure, and the functions which are required in order to achieve proper operation of the antenna at the aperture when connected to such a feed structure. In the present case, it is assumed that the feed structure comprises a waveguide 30 suitable for 16.5 GI-Iz, which may consist of a WR-Sl standard waveguide which is 0.51 inches high (in the Y direction of FIG. 1) and 0.2505 inches deep (into the sheet as seen in FIG. 1). The center of the waveguide is therefore 0.2505 inches above the upper surface of the ground plane 22, which may also comprise the lower wall of the waveguide 30. The E-field in this example is perpendicular to the paper.

The standard WR-Sl waveguide section which supports a TE mode has a wave number of propagation constant of 2.46/cm. Conversion to the desired k of 3.525/cm (equation (5)) is achieved in a matching section M by means of a quarter wavelength, thin dielectric structure, which provides a known quarterwave transformer.

The approach utilized in the present invention to provide a substantially exponential aperture distribution is to convert the half-sine distribution of the TB mode in the waveguide 30, as illustrated by the dotted line 32, by means of the property of a dielectric that it tends to attract or draw energy into itself, so as to gradually reshape the distribution (dotted line 34) into substantial exponential form (dotted line 36). In a strict sense, it is of hyperbolic cosine form in the air section of the waveguide. As seen in FIG. 3, a purely exponential distribution simply drops off from the maximum amplitude A, for increasing values of Y, across the aperture 21. However, the present invention provides a distribution more like that illustrated in FIG. 4, which is substantially exponential across the aperture except across that portion of the aperture which subscribes the thickness, t,,, of the dielectric 24 at the aperture. The utilization of the dielectric to concentrate energy within the dielectric causes a substantial portion of energy to emanate from the surface of the dielectric 24 at the aperture 21, and the remaining energy will drop off exponentially therefrom as illustrated in FIG. 4. This conversion is initiated in a symmetrical fashion in the embodiment of FIG. 1 by a mode conversion section which should be at least several wavelengths long, and in the present example is arbitrarily taken to be 1.0 inches long. Thereafter, in-a transition section which extends from the aperture to the mode conversion section, the high energy concentration in the dielectric is caused to lay adjacent to the reference lane 22 so that the remaining energy drops off in the Y direction therefrom, thereby providing an amplitude distribution as illustrated in FIG. 4, which is a closed approximation of the ideal exponential aperture distribution of FIG. 3. Once the mode conversion section has been defined, the transition section is defined by causing the dielectric 24 to have the same dimension as the mode conversion section at the juncture therewith, and to expand linearly to the aperture, with its lower surface making contact withthe ground plane at precisely the aperture. Thereafter, the profile of an upper surface 38 of the an tenna structure is calculated so as to maintain a constant wave number, k, 3,525, as the dielectric material 24 changes in position and thickness along the X 6 axis from X =0 (at the junction of the mode conversion section with the transition section) to X 2 inches (at the aperture), which is an arbitrarily-chosen length of the transition section, at least several wavelengths long.

The longer the transition section, the greater the assurance of a stable wave having a configuration shown in FIG. 4, as the wave reaches the aperture.

The determination of the thicknesses of dielectric material 24 in the three sections (M, mode conversion and transition) is made utilizing the transverse resonance technique, which is described in detail in 3) Collin, hereinbefore and in 4) Weeks, W. L. Electromagnetic Theory For Engineering Applications, pp. 246-259, John Wiley and Son, Inc., NY. 1964. In this technique, at various points along the length of the dielectric material 24 (pertinent values of x), a theoretical slice of the waveguide structure is considered; in the transition section, the slice is assumed to be very thin so that the reference plane 22, the dielectric material 24, and the upper guiding surface 38 may be considered to be physically parallel to each other. The slice is then considered to be a section of transmission line shorted at both ends. The input impedances seen when looking down (2 and when looking up (2 at anar bitrary point along the transverse axis, the summation of which is zero: 2,, 2,, O 9

With reference to FIG. 5, taking a transverse section (thin slice) acros the waveguide, the air below the dielectric can be assumed to be a short-circuited, airfilled transmission linehaving a characteristic impedance Z and a length (1,; the air above the dielectric is assumed to be an air-filled, short-circuited transmission line having a characteristic impedance Z and a length Z, Z (Z cos kd +j Z sin kd/Z cos kd +j Z sin kd) The analysis herein assumes thatsome portion of the dielectric material constitutes the transmission line for which the input impedance Z, is to be determined by equation l0), and it has a load, Z which inturn comprises an air-filled waveguide which is short circuited. In the case of the propagation constant matching section M (FIG. 1) and in the case of the mode conversion section (FIG. 1), writing the equation for the input impedance, Z,, for each of two waveguides commencing at the center of the dielectric material as seen in FIG. 5, results in only two transmission lines, the summation of impedances of which equal zero, and since these are symmetrical at any point along the matching section and at any point along the mode conversion section, for these two sections the analysis can assume, from symmetry, that these impedances are equal all along the two sections, and that therefore either one of them has an input impedance of zero:

Writing the expression of the input impedance when that input impedance equals zero simply means that the numerator of equation must equal zero:

multiplying by l/sin kd,

Z,=j Ztan kd where Z (wp /k) In this case, since we are considering transverse sections of the antenna structure, the wave number, k for a transmission line in a transverse dimension must be utilized, as defined in equation (10). Where, as in this case, the transmission line has an air dielectric, the relative dielectric constant e, is one, so that kt z 2 1/2 As is known, and from equation (3),

k m(,u. e,,)" 3.456/cm For the transmission line represented by Z d; in FIG.

5, in the matching sect ion, from equations (13) and Z 0 (j Z tan k d cot k (d /2) +j Z2 (jwmlka) tan s a) Cot 2 z/ +j m/k2) 0 The thickness of the dielectric in the matching section M is determined by a wave number or propagation constant desired for a quarterwave transformer, which is known to be simply the square root of the product of the waveguide wave number k 2.46 and the desired aperture wave number k 3.525. Thus From equations (3), (8) and (20), the transverse wave number, k in the matching section dielectric (e,= 4.0) is k [4 (3.458/cm) (2.945/c) 6.25/cm,

and the transverse wave number, k in the air of the matching section is k;, [(3.456/cm) (2.945/cm)] 1.79/cm From FIG. 1,

so that d can be found by solving equation 19), using values of equations (21 (22) and (23), and is found to be d 0.043 cm 0.017 inch. 4

Since A, 2.13 cm,

so the length, X,,,, of the matching section must be a quarter wavelength,

X A /4 0.533 cm 0.21 inches.

For the mode conversion section, the solution is exactly the same except for the fact that the longitudinal wave number in the mode conversion section is taken to be the wave number of the aperture, so that the transverse wave numbers are: k [4(3.456) (3.525) 5.95/cm (2s and k [(3.456 (3.525 =jo.707/cm.

Thus, the thickness of the dielectric, d in the mode conversion section is solved from equations 19), (21 (28) and (29), and is found to be d 0.097 cm 0.0382 inch.

In the transition section, the dielectric varies from a thickness of 0.097 cm determined by equation (30) to a thickness of 0.28 cm as determined from FIG. 2, hereinbefore. The upper and lower surfaces of the dielectric are straight lines centered around a point in the center of the waveguide at the narrow end of the transition section, and such as to cause the lower surface of the waveguide to touch the ground plane at the aperture, as described hereinbefore. Thus the distances at, and d are known for any point along the transition section. The distance d;, is calculated so as to give a constant wave number of k 3.525 all along the transition section. In order to do this, it is apparent that the upper and lower air sections do not have the same length, and therefore do not have the same characteristic impedance as one another for various positions along the waveguide. Therefore equation (11) does not apply, and the more general expression of equation must be solved. In order to do this, 2,, is taken to include the dielectric Z d and the air above the dielectric Z d such that, from equation (10), for the transition sectron,

Z Z (Z cos k d +j Z sin 10 d,

Using the relationships of equations (5) and 16), the transverse wave numbers are and similarly k =ja,

and k k,, (a 1) a The combination of equations (9), (31), (32), (33), (34) and (35) can be rewritten and reduced to Since k and a, are known for each value of x along the transition section, these values and equation (36) can be used in equation (3 7) to solve for d; at any value of .r. Results for .r n(0.1") are shown in Table l, for values of .r from .r 0.0 to .r 1.9".

TABLE I x d v H total height inch inch d,+d +d TABLE l-Continued x d, H total height inch inch d,+d,+d

The shape of the surface 38 as it rises abruptly away from the reference plane 22 near the aperture can be defined simply as a smooth curve asymptotically approaching the aperture 21, of a suitable shape for the size of aperture (usually on the order of 3 or 4 inches) chosen in any given implementation of the present invention. Theoretically, the height of the wave surface structure 38 above the reference plane 22 at the aperture is at infinity. This being so, the aperture resembles a slice of a surface wave antenna comprising a dielectric sheet of the same characteristics as the dielectric 24 at the aperture, with a ground plane 22. As a result of the definition of the aperture, and the design analysis given hereinbefore, it can be understood that an exponential amplitude distribution will exist across substantially the entire aperture, varying therefrom only for the distance of the aperture which is adjacent to the end surface of the dielectric 24.

The antenna structure described herein may be formed into an interferometer antenna, in accordance with the teachings of the aforementioned Cheo et a1 application, by constructing a pair of antennas 20, 20 back-to-back, utilizing the same ground plane 22, as illustrated inFIG. 7. The total aperture across both antennas 20, 20' comprises the individual apertures 21, 21. The amplitude distribution of the interferometer antenna is therefore the distribution illustrated in FIG. 4 together with the mirror image thereof relating to the aperture 21 of the lower antenna 20. Such an antenna will have an ideal sensitivity or scale factor as illustrated in the solid line in FIG. 8, but since the individual aperture distributions as seen in FIG. 4 are not ideal as shown in FIG. 3, the antenna is found to have characteristics which are less than ideal, as expressed by the dotted line in FIG. 8. The advantage of this type of antenna is seen to reside in a very steep slope (which is sensitivity or scale factor) at boresight (0 0) but with lower sensitivity for larger angles of 6, such that the result is unambiguous for angles of incidence as great as 6 =-i40.

The antenna in accordance herewith may also be utilized one at a time (as illustrated in FIGS. 1 and in FIG. 9 hereinafter) wherever characteristics of this type are desired.

The embodiment of FIG. 1 is readily analyzed, but does represent a difficult manufacturing problem in obtaining the dielectric 24 with a bend in it, between the mode conversion and transition section. A more convenient form is a straight wedge as illustrated by the dielectric 24a inthe antenna 20a of FIG. 9. The manner of detailed design of such an antenna may take a variety of forms. Forinstance, the function of matching of wave numbers can be performed simply by starting the calculations at the left end (as seen in FIG. 9) utilizing the method determined for the transition section hereinbefore, with the desired wave number being that of the waveguide (2.46/cm), and for subsequent slices, gradually increasing the value of the wave number to the desired value of 3.525/cm, not over a third of the way along the dielectric 24a from the left end, so as to provide a smooth transition to the desired wave number. This of course alters the shape of the upper wave supporting surface 38a so that it will be different from the surface 38 in FIG. 1.

In the embodiments of FIGS. 1 and 9, the transition section is presumed to have a flat reference plane 22 and the dielectric 24 has been assumed to have straight sides. This results in only one change in dimension (the upper surface 38, 38a) with increasing values of X. If desired, d could equal d throughout the transition section which would tend to simplify the calculations. However, that results in a variation in thickness of the reference plane 22 in such a fashion that it would be difficult to arrange two antennas back-to-back with a common reference plane, as illustrated in FIG. 7, and requires two machined surfaces instead of one. If desired, the dielectric 24, 24a may be provided with curved surfaces, though this also complicates the calculation process, and is not seen to perform any important function. It should be noted, however, that the transverse resonance technique described hereinbefore is useful regardless of the ground rules chosen for the ultimate design of the antenna.

Of course other modification impedance matching and load conversion section of a wedge shape could be utilized in place of the parallel-walled sections illustrated in FIG. 1. In fact, it has been found that an antenna will work if modified from that shown in FIG. 1 simply by removing the dielectric portion which is within the matching section and the mode conversion section, and utilizing a truncated dielectric portion exactly as shown in the transition section of FIG. 1. The problem is simply that there is a lower energy transfer, which effectively lowers the value of A (the maximum amplitude of the aperture distribution).

The description herein is given in terms of an antenna fed by a waveguide supporting waves propagating in the TE mode, but it should be understood that a similar structure may be designed for operation with a feed supporting other TE modes or TM modes. Thus, although the invention has been shown and described with respect to preferred embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention. I

Having thus described typical embodiments of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is:

1. An exponential aperture horn antenna comprising:

a reference wave supporting surface terminating at an aperture of the antenna, said aperture being substantially normal to said reference surface;

a second wave supporting surface, including a waveguide portion parallel with said reference surface and a transition portion in which incremental elements of said second surface are parallel with said reference surface but at various distances therefrom, said second surface terminating at said aperture at a point which is several waveguide wavelengths from said reference surface; and

means, including a dielectric material with one of its ends disposed adjacent said reference surface at said aperture, and its other end disposed in substantially the center of the area between said surfaces of said waveguide portion, for concentrating most of the energy in the antenna structure in the dielectric portion and in an exponentially damped wave to the side of the dielectric portion opposite to the said reference surface, whereby said antenna has an exponential aperture distribution across substantially the entire aperture.

2. The antenna according to claim 1 wherein said means including a dielectric member further comprises means for matching the wave number of said waveguide portion to a different desired wave number of the aperture of the antenna, said desired wave number relating to the design frequency of said antenna and the attenuation constant associated with said exponentially damped wave.

3. An antenna according to claim 2 wherein said means including a dielectric member comprises three dielectric portions, a first portion comprising substantially a truncated wedge and extending from the aperture adjacent said reference surface and joining a second portion thereof having parallel surfaces and being disposed midway in said waveguide portion, said second portion joining a third portion which comprises a quarterwave transformer for matching the waveguide wave number to the aperture wave number.

4. The antenna according to claim 1 wherein said means including a dielectric member comprises a dielectric wedge with the base of said wedge being disposed at said aperture in proximity with said reference surface, and the apex of said wedge being disposed in the center of said waveguide portion at or near the junction with said transition portion, and where a substantial portion of said surfaces are separated by distances which provide the desired aperture wave number relating to portions of said wedge disposed adjacent said aperture.

5. The antenna acording to claim 4 wherein the distance between said wave supporting surfaces is determined so as to provide a variable wave number which gradually varies from the wave number of said waveguide section at the apex of said wedge to the desired wave number at said aperture, over a small portion of said wedge.

6. An interferometer antenna system comprising a pair of antennas according to claim 1 disposed in backto-back relationship with contiguous reference planes.

' Pagee l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION lnvent r( An-Hwa Soong It is certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 3, equation (1) "A(y)=A e should read Column 6, line 29, change "acres" to across Column 7, line 22, change "2 o Z i 5 Column 7, line 52, change "sect ion" to section should read "k [4(3.45 /cm) (23 6742155 1 6.25am;- Column 8, equation '29) ,"k [(3,456 (3 .53 5) o.7o7eul" should read k [0,450 3 .535) l Column 9, equation (31) "z =z (z cos k dj Z2 sin k d 2 cos k d j z sin-k d L 2 2 2 2 2 should read Z Z u 2 Z2 cos k d J Z3 sln k d column 8, equation (21).,"k 4 3 .45 s 2..94 +5/e) ]{G o.zs u" jo.707cm F ORM PO-105O (10-69) U5COMM-DC 6G376-P69 U45. GOVERNMENT PRINTING orncz: 93 o Page 2 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. v Dated August 97 Inventor(s) AHCHWa SOO-ng It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

MW-WW 2 Column 9, equation (36), 1 k (5 --1) should read a (e 1) Column 11, line 31, change "modification" to modifications Column 11, line 31, after "modifications" insert in the basic design may be made: for instance, 'a combination Signed and sealed this 3rd day of December 1974.

(SEAL) 'Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PC I USCOMM-DC heave-P69 US. GOVERNMENT PRINTING OFFICE: o 

1. An exponential aperture horn antenna comprising: a reference wave supporting surface terminating at an aperture of the antenna, said aperture being substantially normal to said reference surface; a second wave supporting surface, including a waVeguide portion parallel with said reference surface and a transition portion in which incremental elements of said second surface are parallel with said reference surface but at various distances therefrom, said second surface terminating at said aperture at a point which is several waveguide wavelengths from said reference surface; and means, including a dielectric material with one of its ends disposed adjacent said reference surface at said aperture, and its other end disposed in substantially the center of the area between said surfaces of said waveguide portion, for concentrating most of the energy in the antenna structure in the dielectric portion and in an exponentially damped wave to the side of the dielectric portion opposite to the said reference surface, whereby said antenna has an exponential aperture distribution across substantially the entire aperture.
 2. The antenna according to claim 1 wherein said means including a dielectric member further comprises means for matching the wave number of said waveguide portion to a different desired wave number of the aperture of the antenna, said desired wave number relating to the design frequency of said antenna and the attenuation constant associated with said exponentially damped wave.
 3. An antenna according to claim 2 wherein said means including a dielectric member comprises three dielectric portions, a first portion comprising substantially a truncated wedge and extending from the aperture adjacent said reference surface and joining a second portion thereof having parallel surfaces and being disposed midway in said waveguide portion, said second portion joining a third portion which comprises a quarterwave transformer for matching the waveguide wave number to the aperture wave number.
 4. The antenna according to claim 1 wherein said means including a dielectric member comprises a dielectric wedge with the base of said wedge being disposed at said aperture in proximity with said reference surface, and the apex of said wedge being disposed in the center of said waveguide portion at or near the junction with said transition portion, and where a substantial portion of said surfaces are separated by distances which provide the desired aperture wave number relating to portions of said wedge disposed adjacent said aperture.
 5. The antenna acording to claim 4 wherein the distance between said wave supporting surfaces is determined so as to provide a variable wave number which gradually varies from the wave number of said waveguide section at the apex of said wedge to the desired wave number at said aperture, over a small portion of said wedge.
 6. An interferometer antenna system comprising a pair of antennas according to claim 1 disposed in back-to-back relationship with contiguous reference planes. 